Hard disk drives (HDDs) are used in almost all computer system operations. In fact, most computing systems are not operational without some type of HDD to store the most basic computing information such as the boot operation, the operating system, the applications, and the like.
The basic HDD model was established approximately 50 years ago and resembles a phonograph. That is, the HDD model includes a hard disk that is spun by a spindle motor at a standard rotational speed. An actuator moves an attached actuator arm over the spinning hard disk. A suspension arm attached to the actuator arm carries a slider. The slider carries a head assembly that includes a magnetic read/write transducer or head for reading/writing information to or from any desired location on the hard disk.
In operation, the hard disk is rotated at a set speed via a spindle motor assembly having a central drive hub. Additionally, there are tracks evenly spaced at known intervals across the hard disk. When a request for a read of a specific portion or track is received, the HDD aligns the head over the specific track location via the actuator arm. Once aligned, the head reads the information from the hard disk. In the same manner, when a request for a write of a specific portion or track is received, the HDD aligns the head over the specific track location and the head writes the information to the hard disk.
Over the years, HDDs have undergone great reductions in size and increases in hard disk rotation speed driven by the need for smaller HDDs used in such devices as personal digital assistants (PDAs), MP3 players, and the like. For example, some of the early HDDs had disk diameters of 24 inches and hard disk rotation speeds of only 1200 rpm, whereas some of the modern disk drives have disk diameters of less than an inch and hard disk rotation speeds of 15,000 rpm. And given the need to maximize the storage capacity of HDDs, the track spacing has also decreased over the years.
As hard disks decrease in size and are required to rotate at higher speeds, problems arise that affect HDD performance, such as non-repeatable runout (NRRO), reliability problems, and acoustical noise problems. NRRO in combination with decreased track spacing can result in track mis-registration. Reliability and acoustical noise are also important issues, especially as the HDDs make their way into smaller consumer devices.
Initially these issues were addressed by replacing the ball bearing spindle motors with fluid dynamic bearing (FDB) spindle motors. Under optimal operating conditions, FDBs used in HDDs produce about 0.01 micro-inches of NRRO, which is about one-tenth the amount produced by similar ball bearings. FDBs are also more reliable and produce less acoustical noise given that there is no metal-to-metal contact between the rotor and stator, as is the case with ball bearing-based spindle motors.
FDBs rely upon the pressures generated in the lubricating fluid by the rotation of the rotor with respect to the stator to stabilize and support the rotor during operation. FIG. 2 shows a typical prior art FDB 200 and the pressures generated in lubricating fluid 230 along rotor bearing surface 225 of rotor 220, as shown in graph 205. The fluid gaps between rotor bearing surface 225 and stator bearing surface 215 are shown vastly magnified for clarity. The pressures within the lubricating fluid are generated by grooves on stator bearing surface 215 of stator 210, which are arranged to form individual bearings that are fluid dynamically coupled. FDBs typically consist of two journal bearings and two thrust bearings. FDB 200 has upper journal bearing 240, lower journal bearing 245, upper thrust bearing 250, and lower thrust bearing 255. Both the journal and thrust bearings use herringbone grooves, as shown in FIG. 5. Both journal bearing grooves 510 and thrust bearing grooves 520 are disposed on stator bearing surface 500.
Since FDBs rely upon the lubricating fluid to supply the pressures needed to operate, voids or bubbles in the lubricating fluid reduce the stiffness of the bearing. This reduced stiffness can lead to excessive repeatable and non-repeatable runout in the FDB, and possibly even FDB failure. The voids or bubbles may enter the fluid through such means as cavitation, gases coming out of solution, or by air being drawn into the lubricating fluid by the nature of the fluid flow within the bearing. As such, these bubbles or voids may be composed of air, oil vapor, or other gases.
Previously, efforts to remove bubbles from the lubricating fluid involve the use of a recirculation channel. As shown in FIG. 2, the features disposed on stator bearing surface 215 create an axial flow through the journal bearings, upper thrust bearing, and recirculation channel 260, as indicated by the arrows. This axial flow is generated by unbalance section 530 of journal bearing grooves 510, as shown in FIG. 5, and helps some bubbles overcome the pressure gradients that would otherwise trap them. For example, as shown in FIG. 2, any bubbles drawn within regions B to L would ordinarily be trapped in an area of low pressure, such as from point C to point D, from point F to point G, from point I to point J, or at point L.
However, the use of recirculation channels is not without significant drawbacks. For example, recirculation channels are unable to completely purge all trapped air bubbles. As shown in graph 205, pressure gradients still remain within the lubricating fluid that trap smaller bubbles. Also, bubbles may still congregate around the lower thrust bearing as there is no axial flow in this region. Moreover, recirculation channels add significant manufacturing cost to the FDB, and consequently, to all products incorporating the FDB. Therefore, a need exists to more effectively and economically drive bubbles within the lubricating fluid to a desired location that is less detrimental to the performance of the FDB.